Polystyrene foam sheet useful for forming deep drawn articles, a process to produce those articles, and the deep drawn articles

ABSTRACT

A polystyrene foam sheet comprising a polystyrene resin which contains 1 to 15 wt % of a rubber component having a majority of particle sizes less than about 0.45 microns has been found to be superior for forming deep drawn articles. Optionally one or two films may be extrusion coated or laminated to the foam sheet prior to thermoforming. Also disclosed is a method for thermoforming the foam sheet. Preheat the sheet, clamp the sheet between the matched male and female mold members, move the members into final position while applying a vacuum through both members to both sides of the foamed sheet and then chill formed foam to set a final shape.

BACKGROUND OF THE INVENTION

Packaging is a major area for use and consumption of foamed polystyreneresins. This packaging is often fabricated from foamed polystyrene sheetinto deep drawn cups, tubs, bowls, trays and similar articles bythermoforming the foamed sheet. It is desirable to have a foam sheetsuitable for thermoforming cups and other deeply drawn articles in asingle thermoforming operation.

Accordingly, it is an object of this invention to provide a polystyrenefoamed sheet which can be formed efficiently into deeply drawn articlesin addition to providing the thermoformed deeply drawn articles madefrom the polystyrene foamed sheet and optionally at least one integral,high-density skin.

This invention also provides as an object a method of producingthree-dimensionally thermoformed deep drawn articles of a low-densitypolystyrene foam core and optionally at least one integral, high-densityskin.

BRIEF DESCRIPTION OF PRIOR ART PRACTICES

In the past, matched mold thermoforming has been commonly employed toform articles from preformed thermoplastic sheet material, which sheetis initially formed utilizing well known thermoplastic extrusiontechniques. The sheet is subsequently preheated and placed between maleand female mold halves, which, as they close, press and form the sheetinto the desired product shape. Obviously, in such an operation, thematerial distribution of the formed product will depend upon the shapesof the mold halves.

An alternate forming arrangement which may be employed to thermoformplastic sheet includes vacuum thermoforming. A vacuum is applied beneaththe preheated sheet to be formed causing atmospheric pressure to pushthe sheet down into contact with the mold. As the sheet contacts themold it cools and sets in the desired configuration. Usually those areasof the sheet material which reach the vacuum mold member last are thethinnest having been drawn to a greater extent than the remainder of thematerial being formed.

Other prior art thermoforming techniques include a two-stagethermoforming technique whereby, utilizing a plug member, a preheatedplastic sheet is only partially preformed into a desired configurationand, after the preforming step, the thermoforming step is completedwhereby the matched mold members come together to form the desiredfinished article. U.S. Pat. No. 3,825,166 discloses such a formingmethod.

In another example, U.S. Pat. No. 3,141,595 discloses a plastic cup madefrom a laminate of foamed material, such as low density polystyrenehaving a density of approximately 6-10 lbs. per cubic foot, and a highdensity material such as a high impact polystyrene sheet having adensity of approximately 63 lbs. per cubic foot. This cup is providedwith a series of projections which represent thickened sidewall regionshaving a lower density than portions of the sidewall remote from theprojections. U.S. Pat. No. 3,141,595 achieves the thickening andlowering of the density in the sidewall regions as a result of themigration of entrapped gases through ruptured cells in the foamedmaterial at the interface between the foamed, low density material andthe unfoamed high density material. This disclosure does not relate toproviding a continuous uninterrupted outer surface on a sidewall whichis thickened, nor to providing thickened sidewall regions in a singleply cup. In U.S. Pat. No. 4,528,221 there is disclosed a polystyrenefoamed sheet suitable for thermoforming into containers, such as cupsand trays. The foamed sheet must have a polystyrene resin as the baseresin, 1-30% (percent) of a rubber component and 1-20% of a fillercomponent. In addition the foamed sheet must have a bulk density of0.13-0.7 g/cm³ (grams/centimeter cubed)(8.12-43.7 pounds per cubicfeet), a stretch ratio less than 1.25 and an amount of residual blowingagent less than 0.3 mole/kg (moles of blowing agent/kilogram).

One limitation in the prior art practices is the inability to be able toeasily form a deep drawn articles using these techniques.

The conventional approach for making formed articles from foamed orcellular thermoplastics is a two-stage process. In the first stage,foamed sheeting is extruded and collected on rolls. The rolls are storeduntil the second stage, which employs a conventional thermoformingmachine for reheating the material on a progressive basis and forming itin molds through the use of differential air pressure, plungers, orboth, whereupon the formed web is transported to a cutting machine forsevering the formed articles from the selvage. The extrusion operationfor producing the sheet material is thus an entirely separate operation(in relation to time and the utilization of heat energy) from thefabricating operation for forming and cutting the articles.

The conventional two-stage process has many limitations affecting cost,quality control, and operational control. Because of the separation ofthe extrusion and fabricating operations, quality control becomes moredifficult and costly. Defects in the sheeting which are not apparentuntil molding begins can not then be corrected, resulting in therejecting of large quantities of material. Since foam sheeting hasexcellent thermal insulating properties, it is difficult and costly toheat it properly during the fabrication step. With certain types ofthermoplastic foam sheeting, there is a period of aging during whichvolatiles used in the foaming process are evolved and replaced by air.Therefore, careful attention must be paid to the time when the reheatingin the fabrication step takes place, because the residual content of thevolatiles can have an appreciable effect on the final density of theproduct. This necessitates operational controls which further complicatethe manufacturing process. Due to the difficulties in obtaining uniformheat and because of the necessity of waiting until a large percentage ofthe volatiles have evolved from the material, it is not possible to formthe foam sheeting as readily or as deeply as would otherwise be thecase.

Moreover, problems which plague the two-stage process become moredifficult when attempting to thermoform deep drawn articles from foamedthermoplastic having a low-density core covered with an integral skin.It is extremely difficult to reheat the core to the necessary formingtemperature without adversely affecting the skin. The presence of theskin tends to produce uneven reheating of the sheeting, resulting inimperfections in the formed articles. Molecular orientation of the skin,which may be important to the overall strength of the formed product, isreduced or destroyed by reheating. Also, in some instances, deep drawnarticles must be pieced together due to the difficulty of forming aunitary article from a single piece of foam sheet.

Processes developed heretofore have not met the requirements forsuccessful application to deeply drawn low-density foamed thermoplasticarticles.

SUMMARY OF THE INVENTION

A polystyrene foam sheet is a polystyrene resin having 1 to 15 weight %of a rubber component (based on polystyrene resin weight) having amajority of particle sizes less than about 0.45 microns with the foamsheet with the foam sheet having a density of 0.04 to 0.16 g/cm³(21/2-10 lb/ft³) and a thickness of 0.4 to 6.5 mm has been found to besuperior for forming deep drawn thermoformed articles.

A method for thermoforming these deep drawn thermoplastic foam articleshas the steps of preheating the sheet of thermoplastic foam stockmaterial which contains 1 to 15 wt % of a rubber component (based onpolystyrene resin weight) having a majority of particle sizes less thanabout 0.45 microns with the foamed sheet having a density of 0.04 to0.16 g/cm³ (21/2-10 lb/ft³) and a thickness of 0.4 to 6.5 mm, thenclamping said preheated stock material in a fixed position betweenmatched male and female mold members relatively moving said male andfemale mold members into final forming position to stretch said sheetinto the female cavity, applying a vacuum through both the male andfemale mold members to both sides of the foamed sheet, while moving themold members into the final forming position, to help expand the sheetinto conformity with substantially the entire cooperating surfaces ofboth the mold members and then chilling the stock material to set afinal shape.

Also disclosed are the deep drawn articles made from the polystyrenefoam sheet and the method of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a 9,000 X photomicrograph of an impact polystyrene having arubber particle size of 2.8 microns.

FIG. 2 is a 9,000 X photomicrograph of rubber particles in foam sheetcell walls where there are two rubber particle sizes, 0.2 and 1.8microns with eighty-seven percent (87%) being the 0.2 size.

FIG. 3 is a sectional view of a male/female mold pair used to form adeeply drawn thermoformed article, in this instance a cup.

FIG. 4 is a deeply drawn article, a cup, formed using the mold pair ofFIG. 3.

FIG. 5 is a cross-sectional view of the sidewall of FIG. 4.

DETAILED DESCRIPTION

The foamed polystyrene sheet of the present invention is a 0.4 to 6.5 mm(millimeter) thick foam sheet composed chiefly of polystyrene resin.

Preferably, the foamed sheet also has one or two non-foamed resin filmswhich may be extrusion coated or laminated by fusion bonding onto one orboth major surfaces of the foam in those manners generally known in theart. The non-foamed resin film is a 5 to 600 μm (micrometers) thickthermoplastic resin film.

The foamed sheet contains 1 to 15 wt % (based on polystyrene weight) ofa rubber component. Preferably the foamed sheet contains 1 to 10 wt % ofa rubber component, most preferably the foamed sheet contains 1 to 5 wt%of a rubber component. The physical characteristics of the rubbercomponent are critical for the production of the deeply drawn articlesof the present invention.

Commercial high-impact polystyrene (HIPS) and some impact modifiedacrylonitrile-styrene-butadiene (ABS) resins have grafted rubberyparticles of broad size distribution in the range of 1 to 5 micron(1000-5000 nanometers, nm) average particle diameter. Some workersconsider such relatively large particle sizes to be necessary to affordthe best impact properties in aromatic polymer blends, however, particlesizes greater than about 400 nm (nanometers) are highly detrimental toclarity of the blends, due to the sensitivity of visible lightscattering to particle size in this particle size range.

Clarity is not a requirement for a polystyrene foam sheet products andthus previously particle size and particle size distribution of therubber particles was not considered to be an important variable in themaking of foam sheet.

In order to be able to successfully and continuously produce a deeplydrawn thermoformed article, the foam sheet must contain a minimum of atleast one percent (1%), and preferably at least two percent (2%),of arubber component in a polystyrene matrix with the rubber componenthaving specific characteristics. One type of such material is generallyknown as impact polystyrene. The impact polystyrene must have amajority, and preferably greater than seventy percent (70%) percent, ofthe occluded or dispersed rubbery particles with an average particlediameter less than about 0.45 microns and should generally have aconventional core-shell morphology (i.e., a rubber shell or membranearound a core of polystyrene). If larger particles are also used theymust not exceed an average particle diameter of about 2.5 microns. Morepreferably the ratio of small to large particles is at least 80/20(small/large) and most preferably it is 85/15.

FIG. 1 is a photomicrograph of an impact polystyrene having a rubberparticle size of 2.8 microns. Foam which are made from this impactpolystyrene or blends of this impact polystyrene with a polystyrenehomopolymer do not consistently produce deeply drawn thermoformedarticles.

FIG. 2 is a photomicrograph of rubber particles in foam sheet cell wallswhere there are two rubber particle sizes, 0.2 and 1.8 microns witheighty-seven percent (87%) being the 0.2 size. This foam consistentlyproduces deeply drawn thermoformed articles.

The impact polystyrene should have a weight percent rubber of between1-15 weight percent, preferably 1-10 weight percent, rubber based on therubber component, such as polybutadiene. Preferably the weight percentrubber is between seven and ten. The weight average molecular weightM_(w) should be between 100,000 and 300,000. and preferably between150,000 and 200,000. The molecular distribution, M_(w) /M_(n), should bebetween 2.7 to 2.9.

One preferred foam sheet is a blend of thirty percent of an impactpolystyrene and seventy percent of a general purpose polystyrenehomopolymer with a weight average molecular weight of about 325,000 anda melt flow rate of about 1.5 grams/10 minutes, such as for example,STYRON 685D, available from The Dow Chemical Company. More preferablythe foam sheet has twenty percent of the impact polystyrene with theremainder being a general purpose polystyrene.

The foam sheet should have a bulk density of 0.04 to 0.16 g/cm³ (gramsper centimeter cubed) (about 21/2 to about 10 pounds per feet cubed).Preferably the foam sheet has a bulk density of 0.04 to 0.128 g/cm³(about 21/2 to about 8 pounds per feet cubed).

The foamed sheet of this invention exhibits very good thermoformabilitywhen used for deep drawing. It is particularly suitable for producingdeeply drawn cuplike formed parts having a desired strength and a drawratio (b/a, where b is depth and a is the widest diameter) greater than1.0 (i.e. the ratio of the depth to the widest diameter is at least1:1).

The article which is specifically disclosed in this application is adeep drawn cup commonly utilized to contain hot fluids and to preventirritation to the holder thereof. Such cups can be made in standardsizes, such as 6 ounces, 8 ounces and even larger sizes. The foamed,cellular thermoplastic cup can be provided with a high gloss non-porousdensified skin layer on the inner surface, and optionally an outerdensified, high gloss surface, and a low density cellular core. The lipmay be rolled inwardly by suitable lip rolling equipment, such ashelical screw lip rollers presently in common usage.

The polystyrene resin constituting the polystyrene foamed sheet of thisinvention includes polymers made up of styrene-type vinyl monomers suchas styrene, methylstyrene, and dimethylstyrene, and also includescopolymers made up of styrene-type vinyl monomers and other vinylmonomers such as acrylic acid, methacrylic acid or ester thereof,acrylonitrile, acrylamide, methacrylonitrile, and maleic anhydride.

The polystyrene foamed sheet of invention can be prepared byextrusion-foaming the resin composition made up of a polystyrene resinand the specific required quantities of rubber component and, ifrequired, a filler. The above-mentioned rubber component may be addeddirectly, but is usually contained in a high-impact polystyrene which isthen blended with a polystyrene homopolymer. The rubber component in thehigh-impact polystyrene may be present in any amount generally known inthe art, so long as when it is blended with the polystyrene homopolymerthe final rubber component content in the foamed product does not exceedfifteen percent (15%), preferably ten percent (10%) and most preferablyfive percent (5%). The rubber component may include butadiene rubber,ethylene-propylene rubber, styrene-butadiene rubber, and polyethylene.They may be added directly to the polystyrene resin. The rubbercomponent when used as a copolymer component includes such monomers asbutadiene, isoprene, and chloroprene and oligomers thereof. They arecopolymerized at a predetermined molar ratio with polystyrene resin. (Inthe case where a copolymer is used as the polystyrene resin, thecopolymer containing the rubber component becomes a terpolymer.)Preferred for this invention are those high-impact polystyrenes thatutilize a styrene/butadiene copolymer as the rubber component.

If the content of the rubber component is less than one percent (1 wt%), the resulting foamed sheet is not suitable for producing deeplydrawn parts. Cups produced from such a sheet lack strength and areliable to break at the lip. Moreover, such a sheet is insufficient inelongation and in productivity. On the other hand, if the content of therubber component exceeds fifteen percent (15 wt %), there is noadditional benefit in thermoforming deeply drawn articles. Moreover, thefoamed sheet may give off an odor of rubber, and is not suitable forproducing food or drink containers. The filler, which is also often anucleating agent, is effective in improving the appearance and thedimensional accuracy and stability of the formed part. While notabsolutely required, the use of a filler, especially for use as anucleating agent is generally preferred when making foam sheet. If thecontent of the filler is too little, it may be difficult to adequatelycontrol gas and cell characteristics, and consequently to control thethickness of the foam sheet and the thermoformed part. On the otherhand, if the content of the filler is excessive, the resulting foamedsheet is insufficient in elongation at the time of forming, although itis possible to control gas and cells. The content of the filler in thepresent invention, if required, is 0.005 to 1.4 wt %, and preferably thecontent of the filler is 0.005 to 0.9 wt %. Most preferably the fillercontent is about 0.005 to about 0.5 weight percent based on total resinweight.

Common examples of filler include talc, calcium carbonate, volcanic ash,gypsum, carbon black, white carbon, magnesium carbonate, clay, naturalsilica, and other common inorganic fillers and metal powder.

The thickness, bulk density, and draw ratio of the foamed sheet can becontrolled by the amount of the filler used to produce foam sheet.

Foamed sheet thickness is important. If the thickness is less than 0.4mm, the foamed sheet cannot be drawn deeply and the resulting formedpart is insufficient in compression strength. If the thickness exceeds6.5 mm, the formability becomes poor; particularly it is difficult tobalance the side wall thickness and the bottom wall thickness. Thepreferred thickness (including the non-foamed resin film) will be atleast partially dependent on the thermoformed deep drawn article. Thethickness can be controlled by adjusting the slit of the extrusion die.The bulk density should be 0.04 to 0.16 g/cm³. If it is higher than0.16, more resin is required and more heat is required for forming,resulting in an extended forming cycle. On the other hand, if the bulkdensity is lower than 0.04, the foamed sheet is insufficient in strengthand when its sheet is formed,the resultant tends to lack a dimensionalaccuracy. Usually, the preferred bulk density is 0.04 to 0.128 g/cm³.Preferably the bulk density is adjusted by changing the quantity of ablowing agent.

Orientation takes place when the foamed sheet, after being initiallyextruded, is then taken up under tension, usually by being wound onto aroll. Biaxial orientation takes place in the case where a circular dieis used. In such a case the foamed sheet is usually slit and laid flatwhile still under tension before being wound onto a roll. Uniaxialorientation is acceptable, but biaxial orientation is preferred in viewof the strength of the resulting formed parts.

The foamed sheet of this invention is produced by extrusion-foaming thatemploys a volatile blowing agent up to about 20 weight percent based onthe total weight of the composition. The examples of the volatileblowing agent include hydrocarbons having a boiling point of -40 degreesto 45 degrees C. (centigrade), such as propane, butane, isopentane andpentane; and polyfluorocarbon blowing agents, such as1,1,-difluoroethane (HFC-152a); 1,2-difluoroethane (HFC-152);1,1,1,2-tetrafluoroethane (HFC-134a); 1,1,2,2-tetrafluoroethane(HFC-134); 1,1,1-trifluoroethane (HFC143a): and 1,1,2-trifluoroethane(HFC-143); pentafluoroethane (HFC-125), preferably HFC-152a andHFC-134a, and most preferably HFC-152a; chloroflurorcarbon andhydrochlorofluorocarbon blowing agents, such as chlorodifluoromethane(HCFC-22), dichlorodifluoromenthe (CFC-12) and trichlorofluoromethane(CFC-11). Of course, nitrogen, carbon dioxide, other inert gases,hydrocarbons and chemical blowing agents can be used in conjunction withthe polyfluorocarbon blowing agents.

In some cases, carbon dioxide, nitrogen gas, water or a combination ofthese compounds may be used as a blowing agent. Carbon dioxide, usedalone, is a preferred blowing agent.

In any case, after forming the foamed sheet, preferably the cells of thefoam are substantially completely filled with air, making the foam sheetproduced suitable for food contact applications.

The blowing agent may be introduced into the extruder in any mannerconventional in the art.

The quantity of residual gas of the blowing agent or air which hasinfiltrated into the cells in the foamed sheet should be in amount so asto prevent secondary foaming or foam collapse from taking place when thesheet is heated for forming, resulting in the formed part being poor inthe reproduction of mold.

If the quantity of residual gas measured immediately after sheetproduction is excessive or insufficient, degassing or air infiltrationshould be performed by heating the sheet to 40 degrees to 50 degrees C.or by permitting the sheet to stand for a certain period of time. Whenusing carbon dioxide as the blowing agent it may be necessary toallowing the foam sheet to stand for a period of time prior tothermoforming, usually up to about 20 hours, until the incomingatmospheric gases in the air equilibrate with the escaping carbondioxide. If this is not allowed to occur, foam sheet collapse may occurduring thermoforming due to insufficient gas in the foam sheet cells.

The polystyrene foamed sheet thus prepared provides satisfactory formedparts, because the quantity of blowing gas in cells is controlled andthe pressure in cells is not excessive or conversely does not becomenegative. The polystyrene foamed sheet containing 1 to 15 wt % of rubbercomponent is superior in elongation when heated for forming deep drawnarticles. The appropriate amount of the rubber component, with therequired characteristics, makes the foamed sheet of the presentinvention suitable for producing deep drawn thermoformed parts withimproved formability.

While not required, it is desirable to laminate or extrusion coat anon-foamed thermoplastic resin film onto at least one surface of thefoamed sheet in order to improve the elongation of the sheet at the timeof forming and the compression strength of the resulting formed part.This non-foamed resin film is usually a 5 to 600 μm thick film ofthermoplastic resin. This film may be laminated or extrusion coated ontoone or both surfaces of the foamed sheet in any conventional manner. Thethermoplastic resin for the non-foamed film includes, for example,polystyrene, polyethylene, high-impact polystyrene which is a mixture orcopolymer of polystyrene and rubber, polypropylene, and polyethyleneterephthalate. Preferable among them from the standpoint of formabilityare high-impact polystyrene and high-density polyethylene; mostpreferable is high-impact polystyrene. Surprisingly while thehigh-impact polystyrene shown in FIG. 1 would be unsuitable forproducing the foamed sheet of the present invention, it is acceptablefor use as the non-foamed resin film.

If the film thickness is less than 5 μm, there is no improvement inelongation or mechanical strength. If the film thickness exceeds 600 μm,the following disadvantage occurs. That is, when each formed part (suchas a cup) is punched out from a formed sheet, the cells at the lip arecollapsed and become open and the laminated film is peeled from thefoamed sheet. Moreover, an excessively thick film is uneconomical. Apreferable film thickness is 30 to 500 μm. Incidentally, this non-foamedfilm contributes to the printability and gas barrier properties of theresulting thermoformed part.

The non-formed thermoplastic film can be laminated onto the foamed sheetin various ways. For instance, the thermoplastic film may be laminatedonto the foamed sheet in a die by using a co-extrusion die (e.g.,cross-head die). In the other way, the foamed sheet and thethermoplastic film extruded from the separate dies can be continuouslylaminated, or the previously extruded thermoplastic film can belaminated onto the foamed sheet. The lamination may be achieved with anadhesive or by fusion-bonding. A variety of adhesives may be used forlamination, e.g., EVA copolymer and SBR in the form of solution,emulsion, or film.

The polystyrene foamed sheet laminated with a non-foamed resin filmprepared as mentioned above is advantageous in that the elongation ofthe foamed sheet at the time of heating is improved and the compressionstrength of the resulting formed part is also improved. Thus, it isuseful as a sheet stock for forming various products that require highdimensional accuracy, particularly deeply drawn articles (having a drawratio greater than 1) that need high compression strength and sufficientelongation at the time of forming. However, with or without thenon-foamed resin film(s), the foamed sheet of this invention is superiorin productivity and therefore is useful as a sheet stock for volumeproduction of deep drawn formed parts.

The conventional approach for making formed articles from foamed orcellular thermoplastics is a two-stage process. In the first stage,foamed sheeting is extruded and collected on rolls. At this point, onemay laminate one or more films onto the foamed sheeting. The rolls arethen stored until the second stage, which employs a conventionalthermoforming machine for reheating the material on a progressive basisand forming it in molds through the use of differential air pressure,plungers, or both, whereupon the formed web is transported to a cuttingmachine for severing the formed articles from the selvage. The extrusionoperation for producing the sheet material is thus, usually, an entirelyseparate operation (in relation to time and the utilization of heatenergy) from the fabricating operation for forming and cutting thearticles.

The conventional two-stage process has many limitations affecting cost,quality control, and operational control. Because of the separation ofthe extrusion and fabricating operations, quality control becomes moredifficult and costly. Defects in the sheeting which are not apparentuntil molding begins can not then be corrected, resulting in therejecting of large quantities of material. Since foam sheeting hasexcellent thermal insulating properties, it is difficult and costly toheat it properly during the fabrication step. With certain types ofthermoplastic foam sheeting, there is a period of aging during whichvolatiles used in the foaming process are evolved and replaced by air.Therefore, careful attention must be paid to the time when the reheatingin the fabrication step takes place, because the residual content of thevolatiles can have an appreciable effect on the final density of theproduct. This necessitates operational controls which further complicatethe manufacturing process. Because of the difficulties in obtaininguniform heat and because of the necessity of waiting until a largepercentage of the volatiles have evolved from the material, it is notpossible to form the foam sheeting as readily or as deeply as wouldotherwise be the case.

Moreover, problems which plague the two-stage process become even moredifficult when attempting to thermoform deep drawn articles from foamedthermoplastic having a low-density core covered with an integral skin ofthe same material. It is extremely difficult to reheat the core to thenecessary forming temperature without adversely affecting the skin. Thepresence of the skin tends to produce uneven reheating of the sheeting,resulting in imperfections in the formed articles. Molecular orientationof the skin, which may be important to the overall strength of theformed product, is reduced or destroyed by reheating. Continuousprocesses developed heretofore, in which extrusion and fabrication stepsfollow without interruption, have not met the requirements forsuccessful application to deep drawn low-density foamed thermoplastics.

EXAMPLES

Foam sheet was produced by extrusion with 80% of a polystyrenehomopolymer having a weight average molecular weight of about 325,000and a melt flow rate of about 1.5 grams/10 minutes and 20% of ahigh-impact polystyrene resin having 8.5 percent rubber based on theweight of the polystyrene, a weight average molecular weight of 165,000,a melt low of 6.7 grams/per 10 minutes and rubber particle sizes of 0.2microns and 1.8 microns. The ratio of small particles to large particleswas 87/13. The blowing agent used, in an amount of 4.8 weight percent,based on total weight, was chlorodifluoromethane (HCFC-22). Additionally0.9 percent talc was added as a filler.

The foam sheet was 0.135 to 0.140 inches thick (3.43-3.56 millimeter)with a 0.18 mm cell size and a six lb./ft³ (0.10 g/cc) density. The foamsheet was then extrusion coated on one major surface by extruding amolten impact polystyrene onto the foam sheet to form a film. The impactpolystyrene had 8 percent rubber based on the weight of the polystyrene,a weight average molecular weight of 170,000, a melt flow of 8.5grams/per 10 minutes and rubber particle sizes of 2.8 microns. The filmwas extruded at three thicknesses of 0.006, 0.009 and 0.012 inches(0.15, 0.23, 0.30 mm). This sheet was then rolled into rolls and allowedto age.

Then prior to thermoforming a second 0.006 skin layer of the same impactpolystyrene was laminated to the other major foam sheet surface that hadnot been extrusion coated.

Cups were then thermoformed from these samples on a conventionalcontinuous feed thermoformer having a male and female mold pair. As canbe seen in the sectional view of FIG. 3, the male and female mold pairhave been altered to provide a vacuum in both the male and female moldmembers to assist in forming this sheet into a cup.

It has been found that an important element for successful forming inaccordance with the method of the present invention is the specificdesign of the male and female mold members as well as its material ofconstruction. The shape and material of the mold members may control thedistribution of the foam sheet material along the side walls of thearticle being formed. Different materials of construction will result inmarked differences in the distribution of material. Accordingly, moldmember construction material must be individually selected dependingupon the shape of the article being formed and the desired materialdistribution in the formed article. Suitable materials include steel,nylon, aluminum, and syntactic foam, for example. For this article,aluminum is the preferred mold member construction material. It will beunderstood that the method of the present invention is not limited to asingle cavity mold operation, but multiple cavity molds may also beemployed.

The mold pair is made of five pieces. The male mold member 10 is asingle piece. The female mold member 50 has four pieces, the topsidewall piece 70, the top sidewall ring 100, the bottom sidewall piece80 and the convex bottom piece 60. The the top sidewall piece 50, thebottom sidewall piece 80 and the convex bottom piece 60 are heldtogether by four bolts 90. The top sidewall ring is bolted into the topsidewall piece 70 with three equally spaced bolts 110.

Male mold member 10 has four equally spaced vacuum holes 12, whosediameters are 0.020 inches (0.051 millimeters), in the concave bottom ofthe piece, with the four holes forming a square around the central endpoint of a central vacuum channel 14 in the male mold member at thepoint of greatest extension into the female mold member 50. Thirty twoadditional vacuum holes 16 are located at the topmost area 17 in themale mold member of the article to be thermoformed, in this case a cuprim. These two vacuum holes 16 communicate with vacuum channel 18 whichis also in communication with vacuum channel 14, while the other thirtyvacuum holes have a channel about 0.12 inch in diameter, just deepenough to communicate with the holes as opposed to extending completelythrough the male mold member, as does vacuum channel 18.

Female mold piece 60 has three vacuum holes of the same diameter (0.020inches) located in the center 61 of the convex bottom 60 of the femalemold with one hole 62 located at the highest convex point and the othertwo holes placed linearly left and right of the center hole spaced asmall distance apart. All three holes communicate with vacuum channel63. Female mold piece 70 also has eighteen equally spaced holes 77 whicheach communicate with vacuum channel 78 about 0.12 inch in diameter.There are also eighteen additional vacuum holes 79 which communicatebetween the interior and the exterior of the female mold piece 70. Thetop sidewall ring 100 has been slightly oversized so as to produceenough of a gap between the top sidewall piece 70 and the top sidewallring 100 so that the vacuum channels 78 and the vacuum holes 79 areaccessible when reducing pressure. In the bottom sidewall piece 80 ofthe female mold 50, there are thirty two equally spaced vacuum holes 84of 0.020 inch diameter which communicate with an annular groove 86 whichis part of vacuum channel 85. An annular ring 67, with an opening ofabout 0.025 inch, communicates with an annular vacuum channel 68 in theconvex bottom piece 60. The annular vacuum channel 68 and vacuum channel63 are also in communication with the four equally spaced vacuumchannels 85. The annular ring provides a full annular ring vacuum whenthermoforming as opposed to separate and non-interconnected vacuum holesin a ring formation.

The gap between the male mold member 10 and the female mold member mayrange between about 0.01 and about 0.07 inch (0.25-1.78 mm).

This mold pair is then completely placed in in a unit which can be usedto reduce air pressure and obtain a partial vacuum.

Surprisingly, in the mold members of the present invention, vacuum ispresent both above (in the male mold member) and below (in the femalemold member) the foamed sheet which is being thermoformed, as opposed tojust using the vacuum assist to pull the sheet into the female moldmember.

The foam sheet was then thermoformed in a conventional multicavitythermoformer using the previously described specifically designed maleand female mold members.

The foam sheet was first preheated in a preheating area to a softeningpoint temperature so that it can be thermoformed into the desired cupshape.

The foam sheet continued through the thermoformer to the forming postionarea and was clamped in position at the top sidewall ring 100.

The male and female mold members were then moved together into the finalforming position to stretch the foam sheet around the male mold memberand into the female cavity.

As this was happening the pressure was reduced by about of 25 inches ofmercury (at 60 degrees Fahrenheit) (85 kilopascals) thus applying avacuum to both sides of the foam sheet as it is being stretched into thefinal forming position.

Then the final shape of the foam sheet is set by chilling. The chillingis accomplished allowing the mold members whose temperature is justbelow the softening point (preheat temperature) of the foam sheet toremain in the final postion long enough to reduce the foam sheettemperature below the softening point.

The thermoformed cups produced in Examples had a draw ratio on the orderof about 1.25:1, a total wall thickness of about 0.044 inch (1.12 mm)and also had the following dimensions:

    ______________________________________                                        Height           3.45 inch (87.6 mm)                                          Top Outer Diameter                                                                             2.75 inch (69.8 mm)                                          Bottom Outer Diameter                                                                          2.00 inch (50.8 mm)                                          ______________________________________                                    

The foam sheet sample with the 0.009 inch extrusion coating and 0.006inch skin layer in conjunction with the foam produces a nine ounce cuphaving acceptable wall strength. FIG. 4 is a cross sectional view ofthis cup. As can be seen in FIG. 5, a close-up of the cross-sectionalsidewall shows that the 0.009 inch extrusion coated exterior wall 22 andthe 0.006 inch laminated interior wall 24, as well as the foam layer 26.

Tests were conducted by applying a force to a horizontal cup sidewallone-third the distance from the top of the cup measured from the cup rimon a compression testing machine at a rate of 10 inches (250 mm) perminute. The cup must be held in place horizontally with the cup sidewallplaced between a fixed member and a movable member which are both longerthan the cup diameter at the rim and have a cylindrical surface of atleast 3.2 mm radius which touches the cup sidewall. The value is thentaken at the first yield point, that is the point at which the valuedecreases or remains the same for increasing deflection of the sidewall.This test is intended to imitate the performance of a cup when a user isholding it.

Example 1 is an example of the present invention and is describedpreviously as the example with the 0.009 inch extrusion coating and a0.006 inch laminated coating.

Competitive Example is a foamed bead cup. Usually such cups are made bymolding foam beads into the shape of a cup.

    ______________________________________                                        EXAMPLE 1           0.71 pounds                                               COMPETITIVE EXAMPLE 0.45 pounds                                               ______________________________________                                    

Preferably Example 1 of the present invention would have a rolled rim,as is conventional in the art, which would increase the force necessaryto deflect the side wall. However, even without a rolled rim, Example 1requires a greater force to deflect the side wall than does the foambead cup (Competitive Example) which indicates that the foam cup of thepresent invention has a better crush resistance then the ComparativeExample. Thus, it is more difficult for people to accidently squeeze thecup walls together when holding the cup of the present invention.

In another test thermocouples were attached to the exterior side wall ofa conventional paper cup and the foam cup of the present invention. Whentemperatures at the exterior wall ceased rising, the temperature of theconventional paper cup was 174 degrees Fahrenheit (°F.) versus 156° F.for the cup of the present invention. The difference in exterior walltemperature is significant when a person holds each cup.

In another series of Examples foam sheet similar to the previousexamples was prepared without any coatings or laminations and was alsothermoformed into cups having a draw ratio greater than 1:1.

It is to be understood that the foregoing description is merelyillustrative of preferred embodiments of the invention, of which manyvariations may be made by those skilled in the art within the scope ofthe following claims without departing from the spirit thereof.

What is claimed is:
 1. A method of thermoforming deep drawnthermoplastic foam articles comprising, in sequence, the steps of:a.preheating a sheet of thermoplastic foam stock material which contains 1to 15 wt % of a rubber component having a majority of particle sizesless than about 0.45 microns with the foamed sheet having a density of0.04 to 0.16 g/cm³ (21/2-10 lb/ft³) and a thickness of 0.4 to 6.5 mm; b.clamping said preheated stock material in a fixed position betweenmatched male and female mold members; and c. relatively moving said maleand female mold members into final forming position to stretch saidsheet into said cavity; d. applying a vacuum through both the male andfemale mold members to both sides of the foamed sheet, while performingstep c), to help expand the sheet into conformity with substantially theentire cooperating surfaces of the mold members; and e. chilling thestock material to set a final shape.
 2. A method of thermoforming deepdrawn thermoplastic foam articles, as recited in claim 1, wherein thedraw ratio is greater than 1/1.
 3. A method of thermoforming deep drawnthermoplastic foam articles, as recited in claim 1, wherein the sheethas a non-foamed film on at least one major surface.
 4. A method ofthermoforming deep drawn thermoplastic foam articles, as recited inclaim 1, wherein the sheet has a non-foamed film on both major surfaces.5. A method of thermoforming deep drawn thermoplastic foam articles, asrecited in claim 1, wherein the sheet has a non-foamed film has athickness of 5-600 μm.
 6. A method of thermoforming deep drawnthermoplastic foam articles, as recited in claim 1, wherein the sheethas a non-foamed film has a thickness of 5-600 μm.
 7. A method ofthermoforming deep drawn thermoplastic foam articles, as recited inclaim 1, wherein the non-foamed film is a polystyrene, polyethylene,high-impact polystyrene, polypropylene or polyethylene terephthalatefilm.
 8. A method of thermoforming deep drawn thermoplastic foamarticles, as recited in claim 1, wherein the non-foamed film is apolystyrene or high-impact polystyrene film.
 9. A method ofthermoforming deep drawn thermoplastic foam articles, as recited inclaim 1, wherein the non-foamed film is a polystyrene or high-impactpolystyrene film.
 10. A deep drawn article made by the method of claim1.